Nanotube Separation Methods

ABSTRACT

A nanotube separation method includes depositing a tag on a nanotube in a nanotube mixture. The nanotube has a defect and the tag deposits at the defect where a deposition rate is greater than on another nanotube in the mixture lacking the defect. The method includes removing the tagged nanotube from the mixture by using the tag. As one option, the tag may contain a ferromagnetic material and the removing may include applying a magnetic field. As another option, the tag may contain an ionic material and the removing may include applying an electric field. As a further option, the tag may contain an atom having an atomic mass greater than the atomic mass of carbon and the removing may include applying a centrifugal force to the nanotube mixture. Any two or more of the indicated removal techniques may be combined.

RELATED PATENT DATA

This patent resulted from a divisional of U.S. patent application Ser.No. 13/276,150 which was filed Oct. 18, 2011, which resulted from adivisional of U.S. patent application Ser. No. 11/713452 which was filedMar. 2, 2007 which is incorporated by reference in its entirety.

TECHNICAL FIELD

Nanotube separation methods, including methods for separating nanotubeshaving a defect.

BACKGROUND

Nanotubes are well known structures exhibiting useful structural,electrical, thermal, and other properties presently of interest in awide variety of technology areas. Nanotubes may exhibit a variety ofintrinsic conductivity states. Fabrication techniques may producesingle-wall nanotubes (SW-NT) and/or multiwall nanotubes (MW-NT).Fabrication techniques may also produce nanotubes of varying diameterand/or length. Further, fabrication techniques may produce a variety ofchiralities (zigzag, armchair, and chiral). In addition to theircomposition and perhaps other physical properties, the listedcharacteristics can influence whether a nanotube is “metallic” (that is,conductive), semiconductive, or insulative.

For some applications, the electrical properties of nanotubes may be ofsmall consequence. However, for other applications, consistentelectrical properties between nanotubes may be desired. A difficultyexists in sorting nanotubes according to their electrical or physicalproperties and/or controlling fabrication methods to produce selectedproperties. In addition to variance of intrinsic properties betweencertain types of nanotubes, variance of properties may exist within thesame type of nanotube due to defects resulting from a disruption in thepattern of chemical bonds often present in a nanotube. Such defects maypotentially change conduction or otherwise have significant implicationsin some uses for nanotubes.

A variety of techniques are under investigation to sort nanotubesaccording to their electrical or other properties or to purify nanotubemixtures, removing the byproducts, such as catalysts, particles, etc.,of nanotube formation methods. However, conventional purification doesnot address defective nanotubes and they may remain in purified nanotubemixtures. Even though, techniques exist for identifying the existenceand location of nanotube defects, manual sorting remains as the onlyoption for separating nanotubes identified as defective from nanotubeslacking defects. Clearly then, a need exists in the art for betternanotube separation methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 illustrate application of various nanotube separation methods.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Ian Salusbury, Finding Fault With Carbon Nanotubes, Materials Today,vol. 9, No. 1-2, Jan.-Feb. 2006, page 9, reports techniques forelectrochemically depositing materials at defect sites of carbonnanotubes to determine the prevalence of defects. Similarly, H. Dai, etal., Electrical Transport Properties and Field Effect Transistors ofCarbon Nanotubes, NANO: Brief Reports and Reviews, vol. 1, No. 1, 2006,pgs. 1-13 reports the failure of atomic layer deposition (ALD) ofdielectric materials to nucleate on nanotubes except at defect sites.Such methods and others that may be appreciated from the subject matterdescribed herein may be used to tag a nanotube having a defect. Thetagged nanotube may then be removed.

Within the context of the present document, a nanotube “defect” refersto a condition in the nanotube structure that gives rise to a reactivesite more susceptible to deposition of a tag in comparison to otherportions of the nanotube lacking the defect. Normally, nanotubes exhibita uniform pattern of chemical bonds. As one example, a defect mayinclude the dangling bonds associated with a disruption in the bondpattern, although, other possibilities are conceivable.

A nanotube separation method according to an embodiment of the presentspecification includes depositing a tag on a nanotube in a nanotubemixture. The nanotube has a defect and the tag deposits at the defectwhere a deposition rate is greater than on another nanotube in themixture lacking the defect. The method includes removing the taggednanotube from the mixture by using the tag. By way of example,depositing the tag may include chemisorbing the tag at the defect wherea chemisorption rate is greater than on another nanotube in the mixturelacking the defect. To increase differentiation between defectivenanotubes and nanotubes lacking a defect, the method may include notchemisorbing the tag on the other nanotube lacking the defect.

The specific structure of the tag desired may depend on the nature ofthe removal process intended. As one option, the tag may consist of asingle monolayer of molecules chemisorbed at the defect. Such singlemonolayer of molecules may be adequate to interact with a subsequentremoval technique, accomplishing the desired separation. In othercircumstances, chemisorbing the tag may include ALD of the tag. Forexample, the tag may include a plurality of monolayers.

ALD involves formation of successive atomic layers on a substrate. Suchlayers may comprise an epitaxial, polycrystalline, amorphous, etc.material. ALD may also be referred to as atomic layer epitaxy, atomiclayer processing, etc. Described in summary, ALD includes exposing aninitial substrate to a first chemical precursor to accomplishchemisorption of the precursor onto the substrate. Theoretically, thechemisorption forms a monolayer that is uniformly one atom or moleculethick on the entire exposed initial substrate. In other words, asaturated monolayer. Practically, chemisorption might not occur on allportions of the substrate. Nevertheless, such an imperfect monolayer isstill a monolayer in the context of this document. Accordingly, a tagchemisorbed at a nanotube defect may thus include a saturated monolayerat the defect. Alternatively, chemisorption might not occur on allportions of the defect.

The first precursor is purged from over the substrate and a secondchemical precursor is provided to react with the first monolayer of thefirst precursor. The second precursor is then purged and the steps arerepeated with exposure of the deposited monolayer to the firstprecursor. In some cases, the two monolayers may be of the sameprecursor. As an option, the second precursor can react with the firstprecursor, but not chemisorb additional material thereto. As but oneexample, the second precursor can remove some portion of the chemisorbedfirst precursor, altering such monolayer without forming anothermonolayer thereon. Also, a third precursor or more may be successivelychemisorbed (or reacted) and purged just as described for the first andsecond precursors.

In the context of the present document, “reacting” or “reaction” refersto a change or transformation in which a substance decomposes, combineswith other substances, or interchanges constituents with othersubstances. Thus, it will be appreciated that “chemisorbing” or“chemisorption” is a specific type of reacting or reaction that refersto taking up and chemically binding (a substance) onto the surface ofanother substance.

ALD is often described as a self-limiting process, in that a finitenumber of sites exist on a substrate to which the first precursor mayform chemical bonds. The second precursor might only bond to the firstprecursor and thus may also be self-limiting. Once all of the finitenumber of sites on a substrate is bonded with a first precursor, thefirst precursor will often not bond to other of the first precursoralready bonded with the substrate. This self-limiting property mayenable precursors to bond to a nanotube defect without bonding to otherportions of a nanotube. A few examples of materials that may bedeposited by ALD include metals, metal oxides, metal nitrides, andothers.

The general technology of chemical vapor deposition (CVD) includes avariety of specific processes, including, but not limited to, plasmaenhanced CVD and others. CVD is often used to form non-selectively acomplete, deposited material on a substrate. One characteristic of CVDis the simultaneous presence of multiple precursors in the depositionchamber that react to form the deposited material. Such condition iscontrasted with the purging criteria for traditional ALD wherein asubstrate is contacted with a single deposition precursor thatchemisorbs to a substrate or reacts with a previously depositedprecursor.

An ALD process regime may provide a simultaneously contacted pluralityof precursors of a type or under conditions such that ALD chemisorption,rather than CVD reaction occurs. Instead of reacting together, theplurality of precursors may chemisorb to a substrate or previouslydeposited precursor, providing a surface onto which subsequentprecursors may next chemisorb or react to form a complete layer ofdesired material. For example, the plurality of precursors may chemisorbto the defect of a nanotube without the precursors reacting with oneanother. In this manner, multiple, different tags may be deposited at ananotube defect.

Under most CVD conditions, deposition occurs largely independent of thecomposition or surface properties of an underlying substrate. Bycontrast, chemisorption rate in ALD might be influenced by thecomposition, crystalline structure, and other properties of a substrateor chemisorbed precursor. Other process conditions, for example,pressure and temperature may also influence chemisorption rate. Incomparison to the predominantly thermally driven CVD, ALD ispredominantly chemically driven. Accordingly, ALD is often conducted atmuch lower temperatures than CVD.

Known process conditions for ALD may yield the desired results describedherein. Further, it is conceivable that CVD conditions might beidentified which result in selective deposition at a nanotube defect.Such a deposition, or other selective deposition techniques, may be usedin the embodiments of the invention.

Chemical and/or physical properties of a tag may be selected thatcorrespond with a certain removal technique. As one option, the tag maycontain a ferromagnetic material and the removing may include applying amagnetic field. As another option, the tag may contain an ionic materialand the removing may include applying an electric field. As a furtheroption, the tag may contain an atom having an atomic mass greater thanthe atomic mass of carbon and the removing may include applying acentrifugal force to the nanotube mixture. Any two or more of theindicated removal techniques may be combined.

Understandably, removal efficiency may vary amongst removal techniques,tags, properties of the nanotubes, and other factors. Consequently,repeating a removal technique and/or using different removal techniquesmay be useful to enhance removal efficiency. It is conceivable that atechnique may remove all of the tagged nanotubes such that the mixtureconsists of untagged nanotubes, but in doing so removes an excessiveamount of untagged nanotubes as well. In such case, the repeated ordifferent removal technique may be applied to the removed mixture oftagged and untagged nanotubes.

In the event that multiple, different removal techniques are used, someconsideration may be given to properties of the tag. A single tag mayexhibit properties that correspond to multiple removal techniques. Inone example, if a tag is selected containing an atom having an atomicmass greater than the atomic mass of carbon and the tag is also ionicand ferromagnetic, then the tag is capable of use in a method thatapplies one or more of a centrifugal force, an electric field, or amagnetic field. In another example, a nanotube may be initially taggedto correspond only to one removal technique and some nanotubes removed.Initial removal may be followed by removal and replacement ormodification of the initial tag to provide another tag or a modified tagwith different properties that correspond to another removal technique.In a further example, multiple, different tags may be deposited at thenanotube defect, the different tags corresponding to different removaltechniques.

Potential ferromagnetic materials that may be provided in a tag includeiron, cobalt, nickel, gadolinium, and dysprosium. Precursors that may beused to deposit ferromagnetic materials include volatile metalacetamidinates, which may be used with H₂ to deposit pure metal. Forexample, [M(RNC(CH₃)NR)₂], where M is Fe, Co, or Ni and R is t-butyl forFe and isopropyl for Co and Ni. Potential materials heavier than carbonthat may be provided in a tag include iron, cobalt, strontium oxide, andstrontium titanate oxide (STO). Notably, the ferromagnetic materialslisted above all have an atomic mass greater than carbon. Precursors, inaddition to the ferromagnetic precursors, that may be used to depositmaterials heavier than carbon include Sr(THD)₂, Sr(DPM)₂, andTi(THD)₂(MPD), where THD is tetramethylheptanedionate, DPM isdipyvaloylmethane, and MPD is methylpentanediolate, with may be usedwith oxidizers such as O₃, H₂O, and/or 0 ₂ to deposit oxides.

Techniques for orienting nanotubes in an electric or magnetic field aredescribed in U.S. patent application Ser. No. 11/107,125 filed Apr. 15,2005 by Sandhu et al. and published Oct. 19, 2006 under Publication No.US 2006/0233694, which functionalizes nanotubes with magnetic orelectrically responsive material. Such functionalizing allowsorganization of nanotubes in response to the electric or magnetic fieldto orient them within a recess prior to shortening the nanotubes. Usingthe electric field essentially amounts to electrophoresis. Sandhu doesnot describe functionalizing nanotubes selectively on the basis of thepresence of a defect. However, utilizing the teachings herein, theapplication Ser. No. 11/107,125 nanotube organization techniques mightbe reconfigured and adapted to instead remove tagged nanotubes as in theembodiments of the present specification.

FIGS. 1-4 illustrate other techniques for removing tagged nanotubes.More specifically, FIGS. 1 and 3 illustrate possible embodimentsincluding applying a magnetic field. In FIG. 1, a fluid 62 within areservoir 40 contains a mixture of nanotubes 48 and ferromagneticallytagged nanotubes 50. Such components represent a suspension device 58,which maintains nanotubes 48 and tagged nanotubes 50 in a state suitablefor use in a separation device 60, also shown in FIG. 1. In the casewhere fluid 62 is a liquid, suspension device 58 may include a stirringmechanism. Also, the nanotubes may be miscible in the liquid to providea uniform distribution. In the case where fluid 62 is a gas, such asair, suspension device 58 may include an air injection apparatus, suchas a fluidized bed, achieving a similar effect. Even though reservoir 40is shown as an open, a closed reservoir may be used, depending on thetype of suspension process, to contain fluid 62 and the nanotubes. Otherknown apparatuses for achieving the purposes of suspension device 58 maybe included.

A flow 52 of fluid 62 containing nanotubes 48 and tagged nanotubes 50enters separation device 60 through a duct 42. Due to the ferromagnetictag, a magnetic field B applied to flow 52 moves tagged nanotubes 50away from some of nanotubes 48 lacking tags. The strength of magneticfield B and the responsiveness of ferromagnetic material in the tag maybe sufficient to move tagged nanotubes 50 away from nanotubes 48 suchthat a flow 54 containing tagged nanotubes 50 enters a duct 44 while aflow 56 of nanotubes 48 enters a duct 46. Understandably, flow 54 maypotentially contain some of nanotubes 48 when they are in alignment withduct 44 upon entering duct 42, since magnetic field B does not changesuch alignment. In contrast, flow 54 may contain primarily, and possiblyentirely, nanotubes 48 given an adequate magnetic response.

Thus, even though flow 54 may contain some of nanotubes 48, taggednanotubes 50 may be removed sufficiently to produce flow 56. Asdescribed, a nanotube separation method may essentially include placingthe nanotube mixture in a fluid, wherein removing the tagged nanotubefrom the mixture occurs while the fluid containing the mixture isflowing through a duct. Those of ordinary skill will appreciate that avariety of alternative configurations and modifications for separationdevice 60 are possible that still rely upon the fundamental principlesdemonstrated in FIG. 1

Instead of a fluid containing the nanotube mixture flowing through aduct, FIG. 3 illustrates another method wherein a fluid 98 in areservoir 90 contains nanotubes 92 and ferromagnetically taggednanotubes 94. A magnet 96 in fluid 98 applies a magnetic field andattracts ferromagnetically tagged nanotubes 94. Removing magnet 96 fromfluid 98 may thus remove tagged nanotubes 94 from fluid 98 by virtue oftheir attraction to magnet 96. Nanotubes 92 and tagged nanotubes 94 maybe suspended in fluid 98 using mechanisms such as those described abovefor suspension device 58 in FIG. 1. Also, fluid 98 may be a gas or aliquid.

FIG. 3 illustrates a method referred to herein as magnetic plating. Inthe same sense that electroplating allows metal ions to plate onto anelectrode, forming a metal layer, a magnetic plating method allowsferromagnetic material to plate onto a magnetized surface, forming alayer of such material. The magnetized surface may be that of apermanent magnet, electromagnet, etc. The magnet may be constructed witha shape that increases its surface area and/or may comprise a porousmaterial, which increases surface area.

In either FIG. 1 or FIG. 3, the mixture of nanotubes in flow 56 or themixture that remains in fluid 98 after removal of magnet 96 containsuntagged nanotubes which lack the defect associated with the tags ontagged nanotubes 50 or 94. Therefore, a nanotube separation methodaccording to an embodiment of the present specification includeschemisorbing a ferromagnetic tag on a nanotube in a nanotube mixture.The nanotube has a defect, the tag chemisorbs at the defect where achemisorption rate is greater than on another nanotube in the mixturelacking the defect, and the tag does not chemisorb on the othernanotube. The method involves placing the mixture in a liquid andremoving the tagged nanotube from the mixture, including applying amagnetic field which uses the tag to move the tagged nanotube away fromthe untagged nanotube. After the removing, the mixture contains theuntagged nanotube.

It is possible that better response of the tagged nanotube to themagnetic field might be obtained by increasing the extent or volume ofchemisorbed material beyond a single monolayer. Consequently,chemisorbing the tag may include atomic layer depositing the tag. Thetag may include a plurality of monolayers.

FIG. 2 illustrates a nanotube separation method analogous to that shownin FIG. 1 except that tagged nanotubes 20 include an ionic material. InFIG. 2, the tags are cationic as indicated by the “+” sign associatedwith tagged nanotubes 20. As may be appreciated, a fluid 32 in areservoir 10 contains nanotubes 18 and tagged nanotubes 20. The nanotubemixture may be suspended with a suspension device 28. A flow 22 ofnanotubes 18 and tagged nanotubes 20 in fluid 32 enters a duct 12. Anelectric field E is applied to flow 22, moving tagged nanotubes 20 awayfrom nanotubes 18 such that a flow 24 enters a duct 14 and containstagged nanotubes 20, potentially along with some of nanotubes 18. Also,a flow 26 containing primarily, and perhaps entirely, nanotubes 18enters a duct 16. Thus, separation occurs within a separation device 30.

Other considerations discussed above with regard to FIG. 1 may alsoapply to FIG. 2. For example, fluid 32 may be a gas or liquid. In theevent that fluid 32 is a gas, the concepts associated with FIG. 2 mayrelate to conventional ion implantation techniques. The primarydifference being that the ions in FIG. 2 are deposited on nanotubes.Techniques in ion implantation for directing a flow of specific ionstoward an implant substrate using an electromagnetic and electrostaticfield might be adapted to direct a flow of ionically tagged nanotubesaway from untagged nanotubes.

FIG. 4 illustrates a nanotube separation method analogous to that shownin FIG. 3. A fluid 82 in a reservoir 70 contains nanotubes 72 andionically tagged nanotubes 74. An anode 78 and a cathode 80 in fluid 82are connected to a voltage source 76. Given the presence of ionic tagson tagged nanotubes 74, tagged nanotubes 74 may be attracted to and“plate” onto anode 78. The technique involves principles related toconventional electroplating, except that tagged nanotubes comprise thematerial plated. Understandably, fluid 82 may be an electrolyte enablingthe described electroplating.

Accordingly, a nanotube separation method in an embodiment of thepresent specification includes chemisorbing an ionic material tag on ananotube in a nanotube mixture, the nanotube having a defect, the tagchemisorbing at the defect where a chemisorption rate is greater than onanother nanotube in the mixture lacking the defect, and the tag notchemisorbing on the other nanotube. The method involves placing themixture in a liquid and removing the tagged nanotube from the mixture,including applying an electric field which uses the tag to move thetagged nanotube away from the untagged nanotube. After the removing, themixture contains the untagged nanotube.

In the described methods that involve applying an electric field,depositing a tag may involve depositing a charge neutral tag, whereinthe tag subsequently becomes ionic, instead of depositing an ionic tag.For example, the methods may further include severing a portion of thetag, leaving an ionic material on the tagged nanotube. The severing maybe initiated by a variety of chemical mechanisms known to those ofordinary skill, including application of optical energy. Alternatively,a portion of the tag as deposited may merely dissociate into a solvent,such as the electrolyte or H₂O, leaving an ionic material on the taggednanotube.

One of the removal techniques mentioned above includes applying acentrifugal force to the nanotube mixture. As one example, centrifugesoperate on well known principles and apply a centrifugal force in orderto separate substances of different specific gravities. A variety ofcentrifuges are known, from familiar laboratory centrifuges toultra-high efficiency centrifuges, which are used to separate elementalisotopes merely by the specific gravity difference arising from the massof a few neutrons. Tagging a nanotube using atoms having an atomic massgreater than the atomic mass of carbon may change the nanotube'sspecific gravity compared to untagged nanotubes, allowing separation byapplying a centrifugal force, such as in a centrifuge. If the tag has asufficiently greater mass, then untagged nanotubes of various lengthsmay still be lighter in comparison and can be separated.

In the case of nanotubes formed from carbon, a nanotube may be taggedwith a tag that contains an atom having an atomic mass greater than theatomic mass of carbon. Thus, the tagged nanotube may exhibit a specificgravity different from that of the other nanotubes in a nanotubemixture. Applying a centrifugal force may consequently move the taggednanotube away from the untagged nanotube.

In a centrifuge, either of the separated fractions of tagged or untaggednanotubes may be removed from the centrifuge, leaving behind the otherfraction. In either manner, the tagged nanotube may be removed from thenanotube mixture. Conventional centrifuges represent suitable devicesfor applying a centrifugal force. However, it is conceivable that otherdevices may be suitable. Selection of a device may depend primarily onoperational efficiency of the centrifuge resulting from specific gravitydifferences determined by the atomic mass of an atom in a nanotube tag.Accordingly, increased separation efficiency may result from anincreased difference in specific gravity. The difference in specificgravity may be increased by utilizing a plurality of the atoms heavierthan carbon and/or atoms that are much heavier. A tag that contains aplurality of atoms having atomic masses that are at least ten times theatomic mass of carbon is expected to allow use of familiar laboratorycentrifuges to separate defective carbon nanotubes.

It will be appreciated that a nanotube separation method according to anembodiment of the present specification includes chemisorbing a tag on acarbon nanotube in a nanotube mixture, the tag containing an atom havingan atomic mass greater than the atomic mass of carbon, the nanotubehaving a defect, the tag chemisorbing at the defect where achemisorption rate is greater than on another carbon nanotube in themixture lacking the defect, and the tag not chemisorbing on the othernanotube. The method involves placing the mixture in a liquid andremoving the tagged nanotube from the mixture, including applying acentrifugal force to the nanotube mixture and using the tag to move thetagged nanotube away from the untagged nanotube. After the removing, themixture contains the untagged nanotube.

In compliance with the statute, the subject matter disclosed herein hasbeen described in language more or less specific as to structural andmethodical features. It is to be understood, however, that the claimsare not limited to the specific features shown and described, since themeans herein disclosed comprise example embodiments. The claims are thusto be afforded full scope as literally worded, and to be appropriatelyinterpreted in accordance with the doctrine of equivalents.

I claim:
 1. A nanotube separation method comprising: depositing a tagcomprising an atom having an atomic mass greater than carbon on ananotube in a nanotube mixture, the nanotube comprising carbon andhaving a defect and the tag depositing at the defect where a depositionrate is greater than on another nanotube in the mixture lacking thedefect; and removing the tagged nanotube from the mixture by using thetag and applying a centrifugal force to the nanotube mixture.
 2. Themethod of claim 1 wherein the atomic mass of the atom is at least 10times the atomic mass of carbon.
 3. The method of claim 1 wherein thedepositing comprises atomic layer deposition.
 4. A nanotube separationmethod comprising: chemisorbing a tag on a carbon nanotube in a nanotubemixture, the tag containing an atom having an atomic mass greater thanthe atomic mass of carbon, the nanotube having a defect, the tagchemisorbing at the defect where a chemisorption rate is greater than onanother carbon nanotube in the mixture lacking the defect, and the tagnot chemisorbing on the other nanotube; placing the mixture in a liquid;removing the tagged nanotube from the mixture, including applying acentrifugal force to the nanotube mixture and using the tag to move thetagged nanotube away from the untagged nanotube; and after the removing,the mixture containing the untagged nanotube.
 5. The method of claim 4wherein the atomic mass of the atom is at least 10 times the atomic massof carbon.
 6. The method of claim 4 wherein the tag consists of a singlemonolayer of the atom chemisorbed at the defect.
 7. The method of claim4 wherein the chemisorbing the tag comprises atomic layer depositing thetag.
 8. The method of claim 7 wherein the tag comprises a plurality ofmonolayers.
 9. A nanotube separation method comprising: tagging aplurality of nanotubes in a nanotube mixture, the tags containing one ormore materials responsive to one or more of a magnetic field, anelectric field, or a centrifugal force, the tagged nanotubes each havinga defect, and a plurality of other nanotubes in the mixture notcontaining a defect and not becoming tagged; applying one or more of themagnetic field, the electric field, or the centrifugal force to thenanotube mixture; moving the tagged nanotubes away from at least some ofthe untagged nanotubes; and removing at least some of the moved, taggednanotubes from the nanotube mixture utilizing centrifuging.
 10. Themethod of claim 9 wherein the tagging comprises atomic layer depositionof one or more materials at the site of the defect.
 11. The method ofclaim 9 where the tag comprises one or more atom having an atomic massgreater than carbon.
 12. The method of claim 9 wherein the tag isadditionally ferromagnetic and a magnetic field is additionally utilizedto separate tagged nanotubes from the mixture.
 13. The method of claim 9wherein the tag is further ionic and an electric field is also used toseparate the tagged nanotubes from the non-tagged nanotubes.
 14. Themethod of claim 9 further comprising, after the removal by centrifuging,modifying the tag on the removed nanotubules and performing a secondremoval process utilizing the modified tag.